1. Field of the Invention
The present invention relates to a method and apparatus for monitoring the planarity of metal sheet, particularly advantageous in the case of steel on discharge from the rolling mill.
2. Description of the Prior Art
The applicant has already advocated a method of this type in which levels are measured with respect to a horizontal plane of reference by means of an optical triangulation. In accordance with this method, this level measurement is carried out for at least two suitably selected longitudinal regions on the surface of the product, the length of these regions is determined by associating the measurement of these levels with the speed of longitudinal displacement of the product, and the differential elongation of these regions is calculated in order to obtain a significant value for the planarity of the said surface.
In an article published in the journal Iron and Steel International, August 1978, p. 215-221, the applicant has described an apparatus comprising three fixed laser beams and three photodiode cameras for measurement of the level of the sheet along the middle region and along the lateral edge regions. These data are supplied to a mini-computer which calculates the planarity values, which values are then displayed on a screen enabling the operators to take the necessary steps with full information.
The results obtained in this way have proved to be very satisfactory, even in the case of very slight planarity defects, which are increasingly common nowadays with the new automatically controlled rolling mills, and it has been possible to monitor not only the quality of the rolled product but also the quality of the rolling work.
However, it is still advantageous to increase the rapidity and the accuracy of these measurements for economic reasons (prevention of rejects) as well as for reasons relating to monitoring efficiency. With the increase in rolling speeds more rapid measurements must be carried out in order to monitor the products during manufacture. In addition, if it is desired to produce long products it is advantageous to repeat the measurements as frequently as possible along the products in order to enable improved localization of the points at which a correct planarity changes to a planarity which is beyond the tolerances.
The positioning of the transmitter (lasers) and receivers (cameras) is very considerable problem which, although only arising once during the rolling of sheets of the same nominal width, arises each time the nominal width of the sheet to be rolled changes, which is the case which occurs most frequently.
In the case of the rolling of wide sheets, it is additionally advantageous to increase the number of measurement points as the area between the middle of the sheet and the edges is greater and requires a greater density of spot measurements, leading consequently to a multiplication of the apparatus components and greater problems with respect to the positioning of this apparatus.
Up to now, various positioning methods have been used, all of which have drawbacks in that they require the displacement of a large number of devices or heavy and large supports, by means which are always complicated and costly. For example, each optical circuit of a measurement triangulation is displaced, this circuit generally being of large size and very heavy.
A further drawback lies in the fact that the perpendicular spacing, which is generally low, left free between the measurement apparatus and the sheet to be monitored, leads to the risk of a collision between the apparatus and a portion of the sheet.